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Specifying thermal components systematically

By Christopher C. Lanham, Watlow

ChemicalProcessing.com

Keywords: "specifying components", "systems approach" and "thermal components"

A common mistake made when specifying a heater or its components is not fully appreciating interactions. Taking a systems approach when specifying thermal components can help.

Interactions are at two levels: between the components and with the environment. Some interactions can be complex. For example, engineers often request heaters with very specific temperature uniformity; power dissipation is an attribute of a heater not uniformity. Temperature uniformity is a characteristic of the interaction between the heater and the environment. Similarly, when it is discovered that a new thermal system design doesn’t control temperature within desired tolerances its often concluded that the accuracy or precision of the temperature control unit must be the problem. What is more likely is that the sensor or heater response is unstable due to mechanical fit or related assembly tolerance problems.

To make the best thermal component choice for any given application, its essential to remember that a system’s performance isn’t simply a sum of its parts. Considering interactions will avoid start-up problems during the checkout of new equipment and improve process reliability during its lifecycle.

Even for very simple thermal systems, taking the time to develop a complete system description before starting detailed design can save time. Many potential headaches such as mismatched components and escalating development costs can be avoided by preventing the quick fixes that are typical for a sketchy system description.

Key components

In its most basic form, a thermal system consists of the following primary components, as illustrated in Figure 1:

Figure 1. A system scheme for temperature control.

• Work load — the fluid, to be heated or cooled (e.g., a liquid or gas);
• Heat transfer medium — the materials and environment that must transfer heat to and from the work load (e.g., a vessel or pipeline);
• Heat/cool source — a device that changes the input power source into heating/cooling energy (e.g., a band heater or chiller unit);
• Process temperature sensor — an instrument that indicates the temperature of the work load (e.g., a thermocouple or infrared pyrometer);
• Process temperature control — manages the temperature of the work load (e.g., a thermostat or electronic control); and
• Power control — connects/disconnects input power to the heat/cool source as determined by the process temperature control (e.g., a thermostat or solid-state relay).

Some thermal systems require additional safeguards. If the system or process is not “inherently safe” (meaning it is capable of posing a hazard to people, equipment or the environment in the event of a malfunction), good thermal system design must  include the following secondary components, also shown in Figure 1:

• Limit temperature sensor — provides a redundant or back-up indication of the temperature of the work load, the temperature of the heat/cool source, or both;
• Limit temperature control — prevents the temperature of the work load from reaching a hazardous level; and
• Safety contactor — an independent means of removing or disconnecting input power from the heat/cool source in the event a hazardous condition occurs.

When schedules are short, design engineers frequently dive right into the details. A specification of a particular component is generated with little effort spent understanding the relationship between other parts. Just as it is the interplay of ingredients that gives “richness” to the flavor of a great bowl of chili, power dissipation across a heater surface is just one ingredient that adds up to temperature uniformity in a thermal system. A heat/cool source can be designed such that the system attribute of temperature uniformity emerges. To do so requires that the heat/cool source be combined with the other system components in a very specific and controlled manner. Defining the relationship of system components such that the desired system performance emerges is the primary challenge when describing a thermal system.

Describing the system

Many different methods have been developed for describing a system or defining system relationships. The important aspects of the most effective approaches are summarized in Figure 2.

Figure 2. Describing the system.

System development should start with a clear statement of need — from the user’s perspective. In the case of a bowl of chili, the corresponding statement of need might be “I want to win the chili cook-off championship.” More relevant to thermal systems, this statement of need might be “I want to maintain the temperature of the reactant flowing in my stainless steel gas delivery line between 170°C and 180°C to prevent it from condensing or degrading.”

Once the need has been clearly established, begin the system description. This document defines how the system will fulfill the need, its behavior: “what it does.” For our cook-off need, one characteristic of behavior is taste — in this case award-winning. For our gas line heating need, one characteristic of behavior is temperature control range — in this case 175 + 5°C. Often behavior is referred to as function and is defined in a functional specification describing the following:

• Mission — objectives to be fulfilled, information to be collected or received, plan or process to be followed and degree of cooperation with others in the environment;
• Viability — the extent the system is able to maintain a separate existence in the environment; and
• Resources — what is required for pursuit of mission and maintenance of viability.